Lipocalins are small secreted proteins that are believed to be involved in the transport of small, hydrophobic molecules. The lipocalin family is characterized by the structural motif of a barrel formed by eight, anti-parallel, beta-sheets, which are arranged as two orthogonal sheets. The lipocalin family is diverse at the sequence level.
The most related members of the family share three characteristic conserved sequence motifs. Members of this group include: retinol-binding protein; purpurin; retinoic acid-binding protein; .alpha..sub.2u -globin; major urinary protein; bilin-binding protein; .alpha.-crustacyanin; pregnancy protein 14; .beta.-lactoglobin; neutrophil lipocalin and choroid plexus protein. Outlier lipocalins are classified as such because they have 2 or less sequence motifs conserved and these proteins include: odorant-binding protein, von Ebner's gland protein, probasin and aphrodisin.
The lipocalins are members of the superfamily known as calycins, all of which are ligand-binding proteins for hydrophobic molecules. Other members of the calycin family are fatty acid-binding proteins (FABPs) and avidins. The members of this super-family share some conformational homology, with little sequence homology (Flower, FEBS Letters 354:7-11, 1994; and Flower, J. Molec. Recognition 8:185-195, 1995).
Von Ebner's gland protein, is also known as tear lipocalin, tear prealbumin or VEGP. It has been shown to be present in the acini of the prostate (Holzfeind et al., FEBS Letters 395:95-98, 1996), acinar cells of the lacrimal glands and von Ebner's gland (Holzfeind et al., Exp. Eye Res. 61:495-500, 1995). VEGP may also be present in salvia, nasal secretions and sweat. VEGP co-localizes with lysomsomes in serous acinar cells and is also present on polyribosomes from the ER and the Golgi apparatus.
Similar to other lipocalins, VEGP is a carrier for retinol or other small hydrophobic compounds. VEGP binds retinol in vitro, and is believed to have an antimicrobial function in the eye, partly because it binds long chain fatty acids which inhibit activation of lysozyme (Glasgow, Arch. Clin. Exp. Ophthalmol. 233:513-522, 1995). The protein may also inactivate enveloped viruses, help surface spreading of the lipid film in the eye and/or protein the epithelium.
Another member of the lipocalin family includes epididymal-retinoic acid binding protein (ERBP), which has tertiary structural homology to retinol-binding protein from human serum (Newcomer et al. J. Biol. Chem. 265:12876-12879, 1990). ERBP is believed to play and important role in maturation of the sperm as it passes through the epididymis. ERBP has been shown to bind a broad spectrum of retinoids, including retinol (vitamin A) retinal, retinyl acetate, .beta.-ionone, cis retinoids, .beta.-carotene, cholesterol, terpenoids, .beta.-lonylideneacetate, long-chain esters of retinol and retinoic acid (Flower, Biochem. J. 318:1-14, 1996) in vivo and/or in vitro. The retinoids have been demonstrated to play important roles in cell differentiation and proliferation, as well as vision, reproductive biology, and mucus secretion. For a review of retinoids and their role in disease and maintenance of homeostasis, see, Goodman, D., N. Engl. J. Med. 310:1023-1031, 1984.
These and other aspects of the invention will become evident upon reference to the following detailed description of the invention and attached drawings.